The Harmonics Generator combines several sections to produce and add 2nd and 3rd harmonics. The possible effects cover the range from brightening up the mix in a way that cannot be achieved using equalizers, to the simulation of the distortion of vintage gear and the colorisation, wrinkling, and/or soiling audio signals in specific frequency bands. The module is a useful tool for special treatment, not only with mastering.

The generator allows adding general 2nd and 3rd harmonics and has two generator sections that are combined with bandpass filters to limit the frequency range of added distortion. The generated harmonics can be added in phase or phase reversed.

This set covers the entire range of conceivable applications. The control ranges are sufficient to accomplish everything between a small colorisation and total distortion. The harmonics generator allows unobstrusive application of the different effects instead of specious and pushing effects. In particular, the addition of third harmonics offers very interesting colorisation effects if used appropriately, since the quint above the octave adds colors that cannot be achieved by 2nd harmonics alone.

The Background

Due to many decades of listening to recorded music, we all got used to the typical side effects of audio recording. Distortion, mainly 2nd and 3rd harmonics a directly connected not only to amplification but to the analog storage methods on disc and tape. The absence of distortion is usually not considered as advantage. Distortion free recordings are described as 'clinical' or 'cold', while an appropriate, small amount of distortion is often considered as 'warm' and 'musical'; however, if the distortion is too high, it is always getting disturbing.

During the analog ages, with analog tape recorders, there was no problem with 'warm' and 'musical' souding mixes; however, there were many other annoying things and problems. Digital audio however, which is free from those effects but has many other bothering problems as well, can benefit from some artificial 'warm' and 'musical' distortion; not all the time, but it's always worth to give it a try. In many cases a mix will sound subjectively better with a little distortion.

In addition, the balance between 2nd and 3rd harmonics is important for the subjetive impression. Two much of one or the other, again results in this 'I'm missing something' feeling.

Die SectionsThe Harmonics Generator U794 has three sections:

General DistortionThe two controls in this section determine the total distortion of the device. The k2 control just adds pure 2nd harmonics that are not frequency depending. The distortion is proportional to the level but not in a linear relation. An appropriately increasing amount of distortion is added over a range of approximately 20 dB. The control law of the k2 pot allows adding small values as well as total distortion at high levels when all to the right. 2nd harmonics are produced by transistors, tubes, and any other kind of active components that can be used for amplifcation. This kind of distortion adds octaves; so it won't sound like distortion as long as the percentage is not too high, since octaves are part of the spectrum of any sound.

The k3 control does two things at a time. A special network that alters the input/output characteristics of the device and produces mainly 3rd harmonics is changed in the influence on the audio signal, depending on the position of the control. If the control is all to the right, a signal is distorted already at an input level of 0 dB. Any higher level causes more distortion, even though the characteristic is designed to increase distortion slowly. When all to the left, no distortion is added. The network simulates the behavior of a pentode but the transisition range from the begin of distortion to teh clipping point is a lot larger, which allows adding distortion more precisely. In addition, the input transformers are affected in a way, that higher levels produce the typical distortion of a bad quality tranformer. This kind of distortion occurs only in the low frequency range, and is caused by the magnetic saturation of the transformer's core. Both effects are combined in a way, that the relation between network and transformer distortion matches the distortion components of a vintage pentode amplifier with transformer. The distortion that is added with the k3 control increases with the level and is higher at low frequencies.

The k2 GeneratorThe harmonics generator for the 2nd harmonics (k2) uses a precise, 4-quadrant, analog multiplier circuit, that squares the input signal. The input of this multiplier is fed thru high-pass and low-pass filters. The low-pass filter has a sweep range from 400 Hz to 20 kHz. An additional switch shifts this range down to 40 Hz to 2 kHz. The sweep range of the high-pass filter from 20 Hz to 2 kHz can by shifted to 200 Hz to 20 kHz with the 'x10' switch. The steepness of the filters is 12 dB/oct. The combination of both filters forms a bandpass filter that allows selecting of any frequency band. The multiplier produces harmonics in the selected frequency band only.

As mentioned above, the multiplier squares the input signal which results in doubling the input frequency. However, it also squares the amplitude the input signal. If the input voltage is 1 V, the output is (1 V * 1V), which is still 1 V. An input of 0.316 V, which is 10 dB less, results in an output voltage of (0.316 V * 0.316 V = 0.1 V). Raising the input voltage to 3 V results in ( 3 V * 3 V = 9 V). These examples make clear that the amount of distortion is extremely depending on the input voltage. A difference of the input level results in a difference of the generated distortion of twice the number of dB's. Using the filters can and will result in huge changes of the level, depending on the setting of the filters and the spectrum of the audio signal. These two facts make necessary to adjust the input select in a way, that the input level of the multiplier is appropriate.

The Drive Control shifts the operation level of the multiplier in the range of +/- 20 dB. When turned to the right, the input level of the multiplier is boosted while the output level is attenuated and vice versa. A LED indicator displays the level at the input of the multiplier. If it is green, the level is too low. Yellow to orange color shows that the level is in a proper range. When the LED turns red, the level is too high and the multiplier clips. Adjusting the drive control appropriately is crucial for proper operation.

The 2nd harmonics at the output of the multiplier can be added to the original signal by the ADD k2 control. The neutral position (no distortion added) is at the center; the range to the right adds distortion in phase while the range to the left adds the harmonics phase reversed. Even though the scaling shifts with the actual level at the input of the multiplier, it gives you an idea about the percentage. It is precise if the LED is orange.

The k3 GeneratorThe 3rd harmonics generator is almost identical to the 2nd harmonics generator. High-pass and low-pass filters as well as the drive control and the LED display are the same; however, the multiplier is a cubing circuit instead of the squaring circuit. Since cubing alters the level much more than squaring, the position of the drive control is more important than in the k2 stage. The 3rd harmonics can be added with the ADD k3 control, in phase or phased reversed, just like described above.

The ModuleThe Harmonics Generator U794 is available with transformer balanced or electronically balanced inputs and outputs. Inputs and outputs can handle levels of + 30 dBu. The unit has a hardbypass that connects inputs and outputs when the device is off.

The U794 is a standard single slot V700 module that fits into all V700 standard frames.